Is an expiatory offering (kaffarah) for breaking an oath obligatory for each instance of masturbation, or a single expiatory offering, for one who covenanted with Allah to abandon it and then broke that covenant?

Masturbation is forbidden, and whoever commits it must repent to Allah. The statement "And Allah is a witness to what I say" is not considered an oath unless the word of covenant is added to it (such as: I pledge to Allah). In that case, it is considered an oath, and if one does not fulfill it, a single expiation for breaking an oath is required, and it is not repeated with repeated breaches.